1. Field of the Invention
The present invention relates to therapeutic ultrasound devices for treating a human body and, more particularly, to controlling the angles at which the acoustic waves are delivered from a transducer to the human body.
2. Related Art
Ultrasound has been used as a therapeutic technique in physical medicine for over 45 years. It has been a recommended treatment technique for adjunctive therapy for the treatment of pain, soft tissue injury, and joint dysfunction including osteoarthritis, periarthritis, bursitis, tenosynovitis, and a variety of musculoskeletal syndromes. Additionally, ultrasound has been used in applications such as acceleration of wound healing, phonophoresis of topical drugs, treatment of scar tissue, and treatment of sports injuries.
The therapeutic biological effects of ultrasound may be characterized into two major areas: thermal and nonthermal. The nonthermal effects can include acoustic streaming, cavitation, and other mechanical effects over the broad range of ultrasonic frequencies from about 0.05 MHz (megahertz) to about 5.0 MHz. The electrical output from a signal generator is converted into mechanical vibration through a transducer which is generally made of a piezoelectric material such as lead zirconate titanate (PZT), single-crystal ferroelectric relaxors such as PMN-PZ-PT, or the like. The mechanical vibration produces an acoustic wave which travels through the tissue and is absorbed in the propagating process. The rate of viscous absorption and the associated increase in temperature are dependent on the micro-structural properties of the tissue-type encountered, the frequency of the acoustic wave, the spatial-temporal acoustic intensity and the degree of nonlinear propagation in tissue. The acoustic energy may be in the form of a continuous wave or a pulsed wave, depending on the therapeutic application, and is typically transferred from the transducer to the patient's tissue using an acoustic coupling material, such as an ultrasonic gel, lotion, hydrogel, or water. Acoustic intensities of 0.03 to 3.0 W/cm2 (Watts per square centimeter) are typically applied for therapeutic purposes, in pulsed or continuous modes, allowing treatment of bone fractures and acute, as well as chronic, tissue injury.
Typically, therapeutic ultrasound treatment is administered by utilizing a piezoelectric transducer normal to the skin tissue interface to generate acoustic longitudinal waves that propagate in tissue, primarily as longitudinal waves, to the treatment area. If the incident longitudinal waves are not normal to the piezoelectric transducer/skin tissue interface, the resulting refracted acoustic waves in the subsequent soft tissue propagate as quasi-longitudinal waves and quasi-shear waves at various refraction angles. As a result, it is often difficult to administer the acoustic waves to patients in the desired alignment with the targeted tissue area using the means for therapeutic ultrasound devices that are currently available.
International Publication No. WO 03/013654A1 taught that shear and longitudinal waves could be controlled and delivered to a tissue by means of a modal converter in the form of a trapezoidal shaped cross-section of a low viscous loss material. The modal converter is a large block of rubber that needs ultrasound coupling gel between the tissue surface area and the rubber block. The shape and design of such a block is difficult to position and restrain on a patient to allow consistent delivery of ultrasound. In addition, the placement of the transducer and the rubber block in relation to the injury requires the center of the block to be offset from the fracture, which is counter-intuitive for most people applying the device to a fracture. The acoustic requirements for this rubber block are such that the material is highly attenuating and requires a much higher incident intensity to be delivered to the first face of the block and has a substantial drain on the battery life of the device and thus on its usability.
Although International Publication No. WO 03/013654A1 explains how to maximize longitudinal and shear waves along the surface of the bone to accelerate periosteal healing, it does not consider the importance of shear waves for the other processes involved in tissue repair. Periosteal direct bone formation is one of the key processes involved in fracture repair, but bone and tissue healing is not limited to only that process. If it is important to provide longitudinal and shear waves to aid specific types of tissue healing, then it would also be important to identify critical angles that result in the desired type of tissue healing.
There remains a need in the art for improved methods and systems for delivering ultrasonic waves to damaged tissue. Further, there remains a need in the art for methods and systems that use ultrasonic waves applied at critical angles to achieve specific types of tissue healing.